Apparatus and methods for cleaning and/or processing delicate parts

ABSTRACT

The invention utilizes harmonics of certain clamped ultrasound transducers to generate ultrasound within the liquid of an ultrasonic tank and in a frequency range of between about 100 khz to 350 khz (i.e., “microsonic” frequencies). The application of microsonic frequencies to liquid preferably occurs simultaneously with a sweeping of the microsonic frequency within the transducer&#39;s harmonic bandwidth to reduce or eliminate (a) standing waves within the liquid, (b) other resonances, (c) high energy cavitation implosion, and (d) non-uniform sound fields, each of which is undesirable for cleaning and/or processing of semiconductor wafers and other delicate parts. The invention can also drive ultrasonic transducers such that the frequency of applied energy has a sweep rate within the ultrasonic bandwidth of the transducers; and that sweep rate is also varied so that the sweep rate is substantially non-constant during operation. This reduces or eliminates resonances which are created by transducers operating with a single sweep rate. An ultrasound generator of the invention sometimes utilizes amplitude modulation (AM), and the AM frequency is swept over time so as to reduce resonances. AM control is preferably provided by selecting a portion of the rectified power line frequency. In applications which utilize multiple generators, multiple transducers, and one or more tanks, simultaneously, the invention synchronizes the operation of the generators to a common FM signal to reduce beat frequencies between generators. Each such generator can also be adjusted, through AM, to control the process characteristics within the associated tank. Two or more transducers are sometimes used by the invention, in combination, to broaden the overall bandwidth of acoustical energy applied to the liquid around the primary frequency or one of the harmonics. The bandwidths of the transducers are made to overlap such that an attached generator can drive the transducers, in combination, to deliver ultrasound to the liquid in a broader bandwidth. In a single chamber ultrasound system, two or more generators, each operating or optimized to generate a different range of frequencies, are connected to a multiplexer; and the desired frequency range is selected, and hence the right generator, according to the cavitation implosion energy that is desired within the tank chemistry.

RELATED APPLICATION

This application is a Divisional Application of Continuation-in-Partapplication Ser. No. 09/066,158, filed Apr. 24, 1998 now U.S. Pat. No.6,181,051, which is a continuation of U.S. patent application Ser. No.08/718,945 filed on Sep. 24, 1996, now U.S. Pat. No. 5,834,871, entitled“Apparatus And Methods For Cleaning And/Or Processing Delicate Parts”,and U.S. Provisional Patent Application Serial No. 60/023,150, filed onAug. 5, 1996, each of which is expressly incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to systems and methods for cleaning and/orprocessing delicate parts, e.g., semiconductor wafers. In particular,the invention relates to ultrasonic systems, ultrasonic generators,ultrasonic transducers, and methods which support or enhance theapplication of ultrasonic energy within liquid.

BACKGROUND OF THE INVENTION

Ultrasonic energy has many uses; and applications of ultrasound arewidespread in medicine, in the military industrial complex, and inengineering. One use of ultrasound in modem manufacturing and processingis to process and/or clean objects within liquids. For example, it iswell-known that objects within an aqueous solution such as water can becleaned by applying ultrasonic energy to the water. Typical ultrasoundtransducers are, for example, made from materials such aspiezoelectrics, ceramics, or magnetostrictives. (aluminum and ironalloys or nickel and iron alloys) which oscillate with the frequency ofthe applied voltage or current. These transducers transmit ultrasoundinto a tank filled with liquid that also covers some or all of theobject to be cleaned or processed. By driving the transducer at itsoperational resonant frequency, e.g., 18 khz, 25 khz, 40 khz, 670 khz or1 Mhz, the transducer imparts ultrasonic energy to the liquid and,hence, to the object. The interaction between the energized liquid andthe object create the desired cleaning or processing action.

By way of example, in the 1970s ultrasonic energy was used in liquidprocessing tanks and liquid cleaning tanks to enhance the manufacture ofsemiconductor devices and other delicate items. The typical ultrasonicfrequency of such processes was a single frequency between 25 khz to 50khz. Many prior art generators exist which produce single frequencyultrasonics, including those described in U.S. Pat. Nos. 3,152,295;3,293,456; 3,629,726; 3,638,087; 3,648,188; 3,651,352; 3,727,112;3,842,340; 4,044,297; 4,054,848; 4,069,444; 4,081,706; 4,109,174;4,141,608; 4,156,157; 4,175,242; 4,275,363; and 4,418,297.

The early ultrasonic transducers were typically piezoelectric ceramicsthat were “clamped,” i.e., compressed, so as to operate at theirfundamental resonant or anti-resonant frequency. Many prior art clampedtransducers exist, including those found in U.S. Pat. Nos. 3,066,232;3,094,314; 3,113,761; 3,187,207; 3,230,403; 3,778,758; 3,804,329 and RE25,433. Other ultrasound transducers are made of alloys that possessmagnetostriction properties which cause them to expand or contract underthe influence of a magnetic field.

As mentioned above, these transducers were bonded to or placed in tankswhich housed the cleaning or processing liquid. Typically, such tankswere constructed of a material compatible with the processing liquid,such as: 316L stainless steel for most aqueous chemistries; 304stainless steel for many solvents; plastics such as Teflon,polypropylene, and metals such as tantalum for strong acids; and coatedmetals such as Teflon-coated stainless steel for corrosive liquids.

In order to deliver ultrasound to the solution within the tank, thetransducers were attached to, or made integral with, the tank. In onemethod, for example, epoxy bonds or brazing were used to attach thetransducers to tanks made of metallic stainless steel, tantalum,titanium, or Hastalloy. In another prior art method, the drive elementsof the transducers were machined or cast into the tank material, and thepiezoelectric ceramic and backplates were assembled to the driveelements.

The prior art also provides systems which utilize ultrasonic transducersin conjunction with plastic tanks. Typically, the tank's plastic surfacewas etched to create a surface that facilitated an epoxy bond thereon.The transducers were bonded with epoxy to the etched surface, andvarious techniques were used to keep the system cool to protect theplastic from deterioration. One such technique was to bond thetransducers to an aluminum plate that would act as a heat-sink, and thento bond the aluminum plate to the plastic surface. Often, fans would bedirected toward the aluminum plate and the transducers so as to enhancecooling. Another cooling technique utilized a thin plastic, or a processof machining the plastic at the trasnducer bonding position, to providea thin wall at the transducer mounting position. This technique enhancedthe cooling of the plastic and transducer by improved heat conductioninto the liquid, and further improved the coupling of sound into theprocessing liquid because of less sound absorption.

With advances in plastic formulations such as PEEK(polyetheretherketone), the prior art made improvements to the plasticultrasonic tank by further reducing the sound absorption within theplastic material. The prior art further developed techniques for moldingthe transducers into the plastic material, such as through injection androtational molding, which further improved the manufacturing of the tankas well as the processing characteristics within the tank.

For other materials such as ceramics, glass, Pyrex and quartz, the priorart used epoxy to bond the transducer to the tank surface. Casting thetransducer into the material was also possible, but was not commerciallyused. Often, the radiating surface (i.e., the surface(s) with theultrasonic transducers mounted thereon or therein), usually the tankbottom, would be pitched by at lease one-quarter wavelength to upsetstanding wave patterns within the tank. Other tank configurations whichprovided similar advantages are reported in the prior art, such asdisclosed by Javorik in U.S. Pat. No. 4,836,684.

An alternative to bonding the transducer directly to the bottom or sidesof the tank was developed in the prior art by bonding the transducer toa window or plate that was sealed within a tank opening via a gasket.This had several advantages. If the transducer failed, or if cavitationerosion occurred within the radiating surface, the window or plate couldbe replaced without the expense of replacing the whole tank. Anotheradvantage was the ability to use dissimilar materials. For example, aquartz tank with a tantalum window offered the advantage of an acidresistant material for the tank, and a metallic bonding and radiatingsurface for the transducer. In U.S. Pat. No. 4,118,649, Schwartzmandescribed the use of a tantalum window with bonded transducers whichcoupled ultrasonic energy into a semiconductor wafer process tank.

A second alternative to direct bonding between the transducers and thetank was developed, in the prior art, by bonding the transducers insidea sealed container, called an “immersible” or “submersible,” which wasplaced under the liquid in the process or cleaning tank. Certainadvantages were also presented in this method, including (a) therelatively inexpensive replacement of the container, and (b) the use ofdissimilar materials, described above. In U.S. Pat. No. 3,318,578,Branson discloses one such immersible where both the transducers and thegenerator are sealed in the container.

There are, however, certain disadvantages associated withabove-described alternatives to direct bonding between the transducersand the tank. One such disadvantage is the occasional entrapment ofcontamination within the area of the window, or the window gasket, orunder the immersible. When contamination-free processing is required, adirect bonded coved comer tank provides a better solution.

Although tanks, plates, windows and immersibles usually had clampedtransducers bonded thereon, the prior art sometimes utilized anunclamped piezoelectric shape or an array of unclamped piezoelectricshapes, such as PZT-4 or PZT-8, which were bonded directly to the tank,plate, window or immersible. By way of example, U.S. Pat. No. 4,118,649describes transducers shaped into hexagons, rectangles, circles, andsquares and bonded to a window. These unclamped transducers had theadvantage of lower cost. They further could be operated in either theradial mode, for low frequency resonance, or in the longitudinal modefor “megasonic” frequency resonance (i.e., “megasonic” frequenciesgenerally correspond to those frequencies between about 600 khz and 2Mhz).

Nevertheless, these prior art unclamped transducers proved to be lessreliable as compared to prior art clamped transducers. Accordingly,these shaped transducer arrays were used primarily in low-cost bench-topultrasonic baths, or in megasonic equipment where high frequencyultrasonic resonance was utilized. Still, these transducers proved to beparticularly unreliable when operating at megasonic frequencies becauseof the high frequency stress affecting the ceramics.

One other system in the prior art used to couple acoustics into a liquidis commonly referred to as a “double boiler” system. In the doubleboiler system, an ultrasonic plate, tank, window or immersible transmitsthe ultrasonics into a coupling liquid. A processing tank, beaker orother container containing the processing or cleaning chemistry is thenimmersed into the coupling liquid. Accordingly, the ultrasound generatedwithin the coupling liquid transmits into the tank containing theprocessing or cleaning liquid. The double boiler system has severaladvantages. One advantage is in material selection: the transducersupport structure can be made out of an inexpensive material, such asstainless steel; the coupling liquid can be a relatively inertsubstance, such as DI water; and the process tank can be a material suchas quartz or plastic material, which fares well with an aggressivechemistry such as sulfuric acid. Another advantage is that onetransducer driving a relatively inert coupling liquid can deliverultrasound into several different processing tanks, each containingdifferent chemistries. Other advantages of the double boiler system arethat the coupling fluid can be chosen so that its threshold ofcavitation is above the cavitation threshold of the processingchemistry; and the depth of the coupling liquid can be adjusted formaximum transmission efficiency into the process tank(s). U.S. Pat. No.4,543,130 discloses one double boiler system where sound is transmittedinto an inert liquid, through a quartz window, and into thesemiconductor cleaning liquid.

The prior art also recognizes multi-functional, single chamberultrasonic process systems which deliver ultrasonic cleaning orprocessing to liquids. In such systems, the cleaning, rinsing, anddrying are done in the same tank. Pedziwiatr discloses one such systemin U.S. Pat. No. 4,409,999, where a single ultrasonic cleaning tank isalternately filled and drained with cleaning solution and rinsingsolution, and is thereafter supplied with drying air. Other examples ofsingle-chamber ultrasonic process systems are disclosed in U.S. Pat.Nos. 3,690,333; 5,143,103; 5,201,958, and German Patent No. 29 50 893.

In the prior art, “directed field tanks” are sometimes employed wherethe parts to be processed have fairly significant absorption atultrasonic frequencies. More particularly, a directed field tank hastransducers mounted on several sides of the tank, where each side isangled such that ultrasound is directed toward the center of the tankfrom the several sides. This technique is useful, for example, insupplying ultrasound to the center of a filled wafer boat.

In the late 1980s, as semiconductor device geometries became smaller,and as densities became higher, many shortcomings were discovered withrespect to conventional low-frequency ultrasonic processing and cleaningof semiconductor wafers. The main disadvantage was that the existingultrasound systems damaged the parts, and reduced production yields. Inparticular, such systems typically generated a sound wave with a singlefrequency, or with a very narrow band of frequencies. In many cases, thesingle frequency, or narrow band of frequencies, would change as afunction of the temperature and age of the transducers. In any event,the prior art ultrasonic systems sometimes generated sufficient cyclesof sound within a narrow bandwidth so as to excite or resonate a mode ofthe processed part. The relatively large displacement amplitudes thatexist during such a mode resonance would often damage the delicate part.

Another disadvantage of single frequency ultrasound (or narrow bandultrasound) is the standing waves created by the resonances within theliquid. The pressure anti-nodes in this standing wave are regions ofintense cavitation and the pressure nodes are regions of littleactivity. Therefore, undesirable and non-uniform processing occurs in astanding wave sound field.

In addition to the resonant and standing wave damages caused by singlefrequency ultrasound (or narrow band ultrasound), damages are alsocaused by (a) the energy levels of each cavitation implosion, and (b) bylower frequency resonances, each of which is discussed below.

The prior art methods for eliminating or reducing the damage caused bythe energy in each cavitation implosion are well known. The energy ineach cavitation implosion decreases as the temperature of the liquid isincreased, as the pressure on the liquid is decreased, as the surfacetension of the liquid is decreased, and as the frequency of the sound isincreased. Any one or combination of these methods are used to decreasethe energy in each cavitation implosion.

By way of example, one benefit in reducing the energy in each cavitationimplosion is realized in the manufacture of hard disk drives forcomputers. The base media for a hard disk is an aluminum lapped andpolished disk. These disks are subjected to 40 khz ultrasonic cleaningin aqueous solutions with moderate temperature, often resulting inpitting caused by cavitation that removes the base material from thesurface of the aluminum disk. As discussed above, one solution to thisproblem is to raise the temperature of the aqueous solution to above 90°C. This causes the energy in each cavitation implosion to be less thanthe energy which typically removes base material from the aluminum disk.It is important, however, to keep the temperature below a value(typically 95° C.) which provides a cavitation implosion that is strongenough to remove the contamination. Another solution to the problem isto use a higher frequency ultrasound. A 72 khz ultrasonic systemtypically has the proper energy level in each cavitation implosion, withmoderate temperature aqueous solutions, to remove contamination withoutremoving base material from the lapped and polished aluminum disk.

In the prior art, wet bench systems often consist of several lowfrequency ultrasonic and/or megasonic tanks with different chemistriesdisposed therein. For example, a cleaning tank followed by two rinsingtanks, usually in a reverse cascading configuration, is a common wetbench configuration. In wet bench systems, there is an optimum value forthe energy in each cavitation implosion: the highest energy cavitationimplosion that does not cause cavitation damage to the part beingprocessed or cleaned. However, because different chemistries are used indifferent tanks in the wet bench system, the energy in each cavitationimplosion, for a given frequency, will be different in each tank.Therefore, not all tanks will have the optimum value of energy in eachcavitation implosion. This problem has been addressed in the prior artby using different frequency ultrasonics in the different tanks. Forexample, the cleaning tank can have a chemistry with low surfacetension, where a low frequency such as 40 khz gives the optimum energyin each cavitation implosion. The rinsing tanks, on the other hand,might use DI water, which has a high surface tension; and thus 72 khzultrasonics may be needed to match the energy of the 40 khz tank foreach cavitation implosion.

In single chamber process systems, different chemistries are pumped inand out of one tank. Because such process systems typically generatesingle or narrow band frequencies, or frequencies in a finite bandwidth,the energy in each cavitation implosion is optimum for one chemistry andnot generally optimum for the other. chemistries. Such systems aretherefore relatively inefficient for use with many differentchemistries.

Certain prior art ultrasonic systems generate ultrasonic frequencies intwo or more unconnected frequencies, such as 40 khz and 68 khz. Althoughthese systems had great commercial appeal, experimental results haveshowed little or no merit to these multi-frequency systems. Such systemstend to have all of the problems listed above, whereby the cleaning anddamaging aspects of ultrasound are generally dependent upon a singlefrequency. That is, for example, if the higher frequency providesadequate cleaning, without damage, the lower frequency may causecavitation damage to the part. By way of a further example, if the lowerfrequency provides cleaning without damage, then the higher frequencyhas little or no practical value.

Cavitation damage can also occur when the delicate parts are removedfrom an operating ultrasonic bath. This damage occurs when theultrasound reflects off of the liquid-air interface at the top of thetank to create non-uniform hot spots, i.e., zones of intense cavitation.The prior art has addressed this problem by turning the ultrasonics offbefore passing a delicate part through the liquid-air interface.

Low frequency resonant damage is a relatively new phenomenon. The priorart has focused on solving the other, more significant problems—i.e.,ultrasonic frequency resonance and cavitation damage—before addressingthe low frequency resonant effects of an ultrasonic system. However, theprior art solutions to low frequency ultrasonic damage are, in part, dueto a reaction to the problems associated with 25 khz to 50 khzultrasound, described above. Specifically, the prior art has primarilyfocused on utilizing high frequency ultrasound in the processing andcleaning of semiconductor wafers and other delicate parts. These highfrequency ultrasonic systems are single-frequency, continuous wave (CW)systems which operate from 600 khz to about 2 Mhz, a frequency rangewhich is referred to as “megasonics” in the prior art.

One such megasonic system is disclosed in U.S. Pat. No. 3,893,869. Thetransducers of this system and other similar systems are typically 0.1inch thick and are unclamped piezoelectric ceramics driven at theirresonant frequency by a single frequency continuous-wave generator. Allthe techniques described above, e.g., material selections, tankconfigurations, and bonding techniques, and used with lower frequencyultrasonics were employed in the megasonic frequency systems of theprior art. For example, because of the aggressive chemistries used,quartz or Teflon tanks with a transducerized quartz window became acommon configuration adapted from lower frequency ultrasonic systems.

As described earlier and disclosed in U.S. Pat. No. 4,118,649, thebonding of piezoelectric shapes to a tank, plate, window or immersible,by epoxy, were the common ways to integrate megasonic transducers withina treatment tank. One alternative is disclosed by Cook in U.S. Pat. No.4,527,901, where the ceramic is fired, and then polarized, as part ofthe tank assembly. Another prior art alternative to the bonding apiezoelectric shape by epoxy is to mold or cast the piezoelectric shapeinto the product. For example, one prior art system utilizes apiezoelectric circle that has been injection-molded into a tankassembly. The prior art also suggests that a piezoelectric rectanglecould be cast into a quartz window; however, in this case, poling orrepoling the ceramic after casting may be necessary if it exceeds itscurie point.

The megasonic systems of the prior art overcame many of thedisadvantages and problems associated with 25 khz to 50 khz systems.First, because the energy in each cavitation implosion decreases withincreasing frequency, damages due to cavitation implosion have beenreduced or eliminated. Instead of cavitation implosion, megasonicsystems depend on the microstreaming effect present in ultrasonic fieldsto give enhanced processing or cleaning. Resonant effects, althoughtheoretically present, are minimal because the geometries of thedelicate parts are typically not resonant at megasonic frequencies. Asgeometries become smaller, however, such as in state-of-the-artequipment, certain prior art megasonic systems have had to increasetheir operating frequencies to 2 Mhz or greater.

An alternative to higher frequency megasonics is to optimize theultrasonic energies with amplitude modulation (AM) of a frequencymodulated (FM) wave. Such systems operate by adjusting one of sevenultrasonic generator parameters—center frequency, bandwidth, sweep time,train time, degas time, burst time, and quiet time—to adjust one or moreof the following characteristics within the liquid: energy in eachcavitation implosion, average cavitation density, cavitation density asa function of time, cavitation density as a function of position in thetank and average gaseous concentration.

When megasonic systems became popular as a solution to cavitation andresonant damages caused by lower frequency ultrasonic systems, the priorart suggests that even higher frequencies be utilized in the removal ofsmaller, sub-micron particulate contamination. Recent data and physicalunderstanding of the megasonic process, however, suggest that this isnot the case. The microstreaming mechanism upon which megasonics dependspenetrates the boundary layer next to a semiconductor wafer and relieson a pumping action to continuously deliver fresh solution to the wafersurface while simultaneously removing contamination and spent chemistry.Cleaning or processing with megasonics therefore depends upon (a) thechemical action of the particular cleaning, rinsing, or processingchemistry in the megasonics tank, and (b) the microstreaming whichdelivers the chemistry to the surface of the part being processed,rinsed, or cleaned.

However, because microstreaming is produced in all high intensityultrasonic fields in liquids, it can be expected that submicron sizeparticle removal will occur in any high intensity ultrasonic field. Whenexperiments were done where the problems of non-uniformity, highcavitation energy, and resonance were overcome by ultrasonic techniquessuch as those taught by U.S. Pat. No. 4,736,130, the data showedeffective submicron particle removal at all ultrasonic frequencies usedfor semiconductor wafer cleaning and processing.

One problem with prior art megasonic systems relates to the transducerdesign and operation frequency. In prior art megasonic systems, thecommonly available 0.1 inch thick piezoelectric ceramic shapes arebonded to a typical tank or gasketed plate and have a fundamentalresonant frequency in the 600 khz to 900 khz frequency range. The maindifference between these megasonic transducers and the 25 khz or 40 khztransducers is that the lower frequency transducers are clamped systems,i.e., where the piezoelectric ceramic is always under compression,whereas the megasonic transducers are unclamped. Because the megasonictransducers are unclamped, the piezoelectric ceramics go into tensionduring its normal operation, reducing the transducer's reliability. Thisremains a significant problem with prior art megasonic systems.

More particularly, ceramic is very strong under compression, but weakand prone to fracture when put into tension. When a clamped transduceris made, the front driver and the backplate compress the piezoelectricceramic by means of a bolt or a number of bolts. However, the frontdriver and the backplate become part of the piezoelectric resonantstructure, and operate to lower the resonant frequency of the combinedpart. The prior art clamped ultrasonic transducer structures resonate atfundamental frequencies well below the megasonic frequencies, andgenerally at 90 khz and below.

Therefore, one significant problem with megasonic systems and equipmentis overall reliability. The megasonic piezoelectric ceramic is put intotension at 600,000 times per second (i.e., 600 khz), at least, duringoperation. This tension causes the ceramics to crack because it weakensand fatigues the material with repeated cycles.

Two other problems of prior art megasonic systems relate to the natureof high frequency sound waves in a liquid. Sound waves with frequenciesabove 500 khz travel like a beam within liquid, and further exhibit highattenuation. This beam effect is a problem because it is very difficultto uniformly fill the process or cleaning tank with the acoustic field.Therefore, the prior art has devised techniques to compensate for thebeam effect, such as by (a) spreading the sound around the tank throughuse of acoustic lenses, or by (b) physically moving the parts throughthe acoustic beam. The beam and attenuation effects of megasonic systemsresult in non-uniform processing, and other undesirable artifacts.

In the last ten years, several manufacturers of prior art ultrasonicsystems have introduced frequency-sweeping ultrasonic generators withcertain frequencies in the 25 khz to 72 khz frequency range. Suchsystems overcome many of the problems associated in the prior art. Byway of example, many or all of the damaging standing waves andresonances are eliminated by these frequency-sweeping ultrasonicsystems. These systems reduce resonant damages by sweeping thefrequencies fast enough, and over a large enough bandwidth, so that itgreatly reduces the likelihood of having resonances within the tank. Arapid frequency sweeping system generates each cycle of sound (or insome cases, each half cycle of sound) at a significantly differentfrequency from the preceding cycle of sound (or half cycle of sound).Therefore, the build up of resonant energy required to impart aresonance amplitude within the part rarely or never occurs.

Another advantage of frequency sweeping ultrasonic systems is that theyincrease the ultrasonic activity in the tank because there is less lossdue to wave cancellation. One of the first frequency sweeping ultrasonicgenerators had a bandwidth of 2 khz, a sweep rate of 100 hz, and acenter frequency of 40 khz. Accordingly, at a frequency change 400 khzper second—i.e., two kilohertz sweeping up from 39 khz to 41 khz, plustwo kilohertz sweeping down from 41 khz to 39 khz, times 100 times persecond equals 400 khz per second—the increased ultrasonic activity wasable to cavitate semi-aqueous solvents which were previously impossibleto continuously cavitate with commercially available conventionalultrasonic generators.

The frequency-sweeping activity in the prior art was so significant thatby 1991 every major ultrasonic manufacturer was shipping 40 khzgenerators that changed frequency at frequency sweep rates of up to 4.8Mhz per second. This rapid sweeping of frequency provided goodultrasonic activity even at continuous wave (CW) operation. By way ofexample, one 10 kilowatt, 40 khz generator in the prior art operateddirectly from a rectified three-phase power signal which provided a 800khz per second CW frequency-sweeping system that had superiorperformance as compared to AM single frequency ultrasonic systems.

Although the main problems with lower frequency ultrasonics were solvedby frequency sweeping, cavitation damage could occur in any processwhere the energy in each cavitation implosion was strong enough toremove an atom or a molecule from the surface of the semiconductor waferor the delicate part. As disclosed in U.S. Pat. No. 4,736,130, systemoptimization at lower frequency ultrasonics permitted successfulprocessing of many delicate parts because it was possible to maximizethe microstreaming effects while minimizing adverse cavitation effects.However, the potential for cavitation damage remains a concern of theindustry.

One important limitation to further improvement of ultrasonic processesis the low frequency and the narrow bandwidth of clamped piezoelectrictransducers. For example, typical clamped or unclamped prior arttransducers provide about 4 khz in overall bandwidth. One otherimportant limitation of ultrasonic processes is that although amplitudecontrol is known to be beneficial, inexpensive and uncomplicated ways ofproviding AM are generally not available.

Other problems exist in the prior art in that certain systems are drivenby more than one ultrasonic generator. Such generators typically operateto either (a) drive the same tank, or (b) drive multiple tanks in thesame system. Although the generators are typically set to the same sweeprate, the independent generators will never have exactly the same sweeprate. This causes another low frequency resonance problem within anultrasonic tank or system. In addition, one problem with multiple tanksand multiple generators is that some of the ultrasound from one tank iscoupled through connecting structure to the other tank(s). This createsunwanted cross-talk and negatively affects the desired cleaning orprocessing within the tank.

In particular, prior art multi-generator systems sometimes create anundesirable beat frequency which causes low frequency resonance insusceptible parts. For example, consider two sweeping frequencygenerators, each with sweep rates of approximately 10 hz sweeping over abandwidth of 4 khz with a center frequency of 40 khz. Now consider adelicate part to be cleaned that has a low frequency resonance at onekilohertz. The following condition will occur periodically: onegenerator will be changing frequency from 38 khz to 41 khz, while theother generator is changing frequency from 39 khz to 42 khz. In thisexample, this will occur for about 37.5 milliseconds. Since the twofrequencies in the tank or system are about one kilohertz apart, a beatfrequency of about one kilohertz is produced. The period of onekilohertz is one millisecond, therefore a string of thirty-seven beatsat about one kilohertz are produced. This is sufficient to setup adestructive resonance in a delicate part with a one kilohertz resonance

It is, therefore, an object of the invention to provide ultrasonicsystems which reduce or eliminate the problems in the prior art.

Another object of the invention is to provide improvements to ultrasonicgenerators, to transducers applying ultrasound energy to liquids, and tomethods for reducing the damage to delicate parts.

It is still another object of the invention to provide methodology forapplying ultrasound to liquid in a manner which is compatible with boththe tank chemistry and the part under process.

Still another object of the invention to provide a method of supplyingsuitable energies in each cavitation implosion, in a single chamberprocess system, where different chemistries are used in different partsof the process.

Another object of the invention is to provide an ultrasonic generatorthat reduces the repetition of low frequency components from anultrasonic bath to reduce or eliminate low frequency resonances withinthe bath.

One objective of this invention is to overcome certain disadvantages ofprior art megasonic systems while retaining certain advantages ofmegasonic cleaning and/or processing.

It is a further objective of this invention to provide ultrasonictransducer arrays which supply ultrasonic energy with microstreaming andwithout significant cavitation implosion.

Still another object of this invention is to provide methodology ofimproved amplitude control in ultrasonic systems.

Another object of the invention is to provide systems which reduce oreliminate beating and/or cross-talk within a liquid caused bysimultaneous operation of a plurality of generators.

These and other objects of the invention will be apparent from thedescription which follows.

SUMMARY OF THE INVENTION

As used herein, “ultrasound” and “ultrasonic” generally refer toacoustic disturbances in a frequency range above about eighteenkilohertz and which extend upwards to over two megahertz. “Lowerfrequency” ultrasound, or “low frequency” ultrasound mean ultrasoundbetween about 18 khz and 90 khz. “Megasonics” or “megasonic” refer toacoustic disturbances between 600 khz and 2 Mhz. As discussed above, theprior art has manufactured “low frequency” and “megasonic” ultrasoundsystems. Typical prior art low frequency systems, for example, operateat 25 khz, 40 khz, and as high as 90 khz. Typical prior art megasonicsystems operate between 600 khz and 1 Mhz. Certain aspects of theinvention apply to low frequency ultrasound and to megasonics. However,certain aspects of the invention apply to ultrasound in the 100 khz to350 khz region, a frequency range which is sometimes denoted herein as“microsonics.”

As used herein, “resonant transducer” means a transducer operated at afrequency or in a range of frequencies that correspond to a one-halfwavelength (λ) of sound in the transducer stack. “Harmonic transducer”means a transducer operated at a frequency or in a range of frequenciesthat correspond to 1λ, 1.5λ, 2λ, or 2.5λ of sound, and so on, in thetransducer stack. “Bandwidth” means the range of frequencies in aresonant or harmonic region of a transducer over which the acousticpower output of a transducer remains between 50% and 100% of the maximumvalue.

As used herein, a “delicate part” refers to those parts which areundergoing a manufacture, process, or cleaning operation within liquidsubjected to ultrasonic energy. By way of example, one delicate part isa semiconductor wafer which has extremely small features and which iseasily damaged by cavitation implosion. A delicate part often definescomponents in the computer industry, including disk drives,semiconductor components, and the like.

As used herein, “khz” refers to kilohertz and a frequency magnitude ofone thousand hertz. “Mhz” refers to megahertz and a frequency magnitudeof one million hertz.

As used herein, “sweep rate” or “sweep frequency” refer to the rate orfrequency at which a generator and transducer's frequency is varied.That is, it is generally undesirable to operate an ultrasonic transducerat a fixed, single frequency because of the resonances created at thatfrequency. Therefore, an ultrasonic generator can sweep (i.e., linearlychange) the operational frequency through some or all of the availablefrequencies within the transducer's bandwidth at a “sweep rate.”Accordingly, particular frequencies have only short duration during thesweep cycle (i.e., the time period for sweeping the ultrasound frequencythrough a range of frequencies within the bandwidth). “Sweep the sweeprate” or “double sweeping” or “dual sweep” refer to an operation ofchanging the sweep rate as a function of time. In accord with theinvention, “sweeping the sweep rate” generally refers to the operationof sweeping (i.e., linearly changing) the sweep rate so as to reduce oreliminate resonances generated at the sweep frequency.

The present invention concerns the applied uses of ultrasound energy,and in particular the application and control of ultrasonics to cleanand process delicate parts, e.g., semiconductor wafers, within a liquid.Generally, in accord with the invention, one or more ultrasonicgenerators drive one or more ultrasonic transducers, or arrays oftransducers, coupled to a liquid to clean and/or process the delicatepart. The liquid is preferably held within a tank; and the transducersmount on or within the tank to impart ultrasound into the liquid. Inthis context, the invention is particularly directed to one or more ofthe following aspects and advantages:

(1) By utilizing harmonics of certain clamped ultrasound transducers,the invention generates, in one aspect, ultrasound within the liquid ina frequency range of between about 100 khz to 350 khz (i.e.,“microsonic” frequencies). This has certain advantages over the priorart. In particular, unlike prior art low frequency ultrasound systemswhich operate at less than 100 khz, the invention eliminates or greatlyreduces damaging cavitation implosions within the liquid. Further, thetransducers operating in this frequency range provide relatively uniformmicrostreaming, such as provided by megasonics; but they are alsorelatively rugged and reliable, unlike megasonic transducer elements. Inaddition, and unlike megasonics, microsonic waves are not highlycollimated, or “beam-like,” within liquid; and therefore efficientlycouple into the geometry of the ultrasonic tank. Preferably, theapplication of microsonic frequencies to liquid occurs simultaneouslywith a sweeping of the microsonic frequency within the transducer'sharmonic bandwidth. That is, microsonic transducers (clamped harmonictransducers) are most practical when there is a sweep rate of theapplied microsonic frequency. This combination reduces or eliminates (a)standing. waves within the liquid, (b) other resonances, (c) high energycavitation implosions, and (d) non-uniform sound fields, each of whichis undesirable for cleaning or processing semiconductor wafers anddelicate parts.

(2) The ultrasound transducers or arrays of the invention typically havea finite bandwidth associated with the range of frequencies about aresonant or harmonic frequency. When driven at frequencies within thebandwidth, the transducers generate acoustic energy that is coupled intothe liquid. In one aspect, the invention drives the transducers suchthat the frequency of applied energy has a sweep rate within thebandwidth; and that sweep rate is also varied so that the sweep rate issubstantially non-constant during operation. For example, the sweep ratecan change linearly, randomly, or as some other function of time. Inthis manner, the invention reduces or eliminates resonances which arecreated by transducers operating with a single sweep rate, such asprovided in the prior art.

(3) At least one ultrasound generator of the invention utilizesamplitude modulation (AM). However, unlike the prior art, this AMgenerator operates by selectively changing the AM frequency over time.In a preferred aspect of the invention, the AM frequency is sweptthrough a range of frequencies which reduce or eliminate low frequencyresonances within the liquid and the part being processed. Accordingly,the AM frequency is swept through a range of frequencies; and this rangeis typically defined as about 10-40% of the optimum AM frequency. Theoptimum AM frequency is usually between about 1 hz and 10 khz.Therefore, for example, if the optimum AM frequency is 1 khz, then theAM frequency is swept through a frequency range of between about 850 hzand 1150 hz. In addition, the rate at which these frequencies are variedis usually less than about {fraction (1/10)}th of the optimum AMfrequency. In this example,. therefore, the AM sweep rate is about 100hz. These operations of sweeping the AM frequency through a range offrequencies and at a defined AM sweep rate reduce or eliminate unwantedresonances which might otherwise occur at the optimum AM frequency. Inanother aspect of the invention, for delicate parts with very lowfrequency resonances, the AM frequency is changed randomly or the AMsweep rate is swept at a function of time with a frequency about{fraction (1/10)}th of the AM sweep rate.

(4) The invention provides AM control by selecting a portion of therectified power line frequency (e.g., 60 hz in the United States and 50hz in Europe). In one aspect, this AM control is implemented byselecting a portion of the leading quarter sinusoid in a full waveamplitude modulation pattern that ends at the required amplitude in thezero to 90° and the 180° to 270° regions. Another AM control isimplemented by selecting a portion of the leading quarter sinusoid in ahalf wave amplitude modulation pattern that ends at the requiredamplitude in the zero to 90° region.

(5) The invention can utilize several tanks, transducers and generatorssimultaneously to provide a wet bath of different chemistries for thedelicate part. In one aspect, when two or more generators are operatingat the same time, the invention synchronizes their operation to a commonFM signal so that each generator can be adjusted, through AM, to controlthe process characteristics within the associated tank. In this manner,undesirable beating effects or cross-coupling between multiple tanks arereduced or eliminated. In a preferred aspect, a master generatorprovides a common FM signal to the other generators, each operating as aslave generator coupled to the master generator, and each slavegenerator provides AM selectively. In addition, because the transducersin the several tanks are sometimes swept through the frequencies of thetransducer's bandwidth, the FM control maintains overall synchronizationeven though varying AM is applied to the several transducers. Themulti-generator FM synchronization also applies to single tankultrasonic systems. That is, the invention supports the synchronizedoperation of a plurality of generators that are connected to a singletank. In this case, each generator has an associated harmonic transducerarray and is driven with a common FM signal and AM signal so that thefrequencies within the tank are synchronized, in magnitude and phase, toreduce or eliminate unwanted resonances which might otherwise occurthrough beating effects between the multiple generators and transducers.

(6) In another aspect, the invention utilizes two or more transducers,in combination, to broaden the overall bandwidth of acoustical energyapplied to the liquid around the primary frequency or one of theharmonics. For example, the invention of one aspect has two clampedtransducers operating at their first, second third, or fourth harmonicfrequency between about 100 khz and 350 khz. The center harmonicfrequency of each is adjusted so as to be different from each other.However, their bandwidths are made to overlap such that an attachedgenerator can drive the transducers, in combination, to deliverultrasound to the liquid in a broader bandwidth. In a preferred aspect,two or more transducers, or transducer arrays, operate at uniqueharmonic frequencies and have finite bandwidths that overlap with eachof the other transducers. If, for example, each transducer has abandwidth of 4 khz, then two such transducers can approximately providea 8 khz bandwidth, and three such transducers can approximately providea 12 khz bandwidth, and so on.

(7) In one aspect, the invention provides a single tank system whichselects a desired frequency, or range of frequencies, from a pluralityof connected ultrasonic generators. Specifically, two or moregenerators, each operating or optimized to generate a range offrequencies, are connected to a mux; and the system selects the desiredfrequency range, and hence the right generator, according to thecavitation implosion energy that is desired within the tank chemistry.

(8) The invention has additional and sometimes greater advantages insystems and methods which combine one or more of the features in theabove paragraphs (1) through (7). By way of example, one particularlyuseful system combines two or more microsonic transducers (i.e.,paragraph 1) to create broadband microsonics (i.e., paragraph 6) withinliquid. Such a system can further be controlled to provide a specificamplitude modulation (i.e., paragraph 4). Other particularlyadvantageous systems and methods of the invention are realized with thefollowing combinations: (2) and (4); (1), (2) and (4); and (1) and (2)with frequency sweeping of the microsonic frequency.

The following patents, each incorporated herein by reference, provideuseful background to the invention in the area of ultrasonic generators:U.S. Pat. Nos. 3,152,295; 3,293,456; 3,629,726; 3,638,087; 3,648,188;3,651,352; 3,727,112; 3,842,340; 4,044,297; 4,054,848; 4,069,444;4,081,706; 4,109,174; 4,141,608; 4,156,157; 4,175,242; 4,275,363; and4,418,297. Further, U.S. Pat. Nos. 4,743,789 and 4,736,130 provideparticularly useful background in connection with ultrasonic generatorsthat are suitable for use with certain aspects of the invention, andare, accordingly incorporated herein by reference.

Clamped ultrasonic transducers suitable for use with the invention areknown in the art. For example, the following patents, each incorporatedherein by reference, provide useful background to the invention: U.S.Pat. Nos. 3,066,232; 3,094,314; 3,113,761; 3,187,207; 3,230,403;3,778,758; 3,804,329 and RE 25,433.

Techniques for mounting or affixing transducers within the tank, and ofarranging the transducer and/or tank geometry are, for example,described in U.S. Pat. Nos. 4,118,649; 4,527,901; 4,543,130; and4,836,684. Each of these patents is also incorporated by reference.

Single chamber ultrasonic processing systems are described, for example,in U.S. Pat. Nos. 3,690,333; 4,409,999; 5,143,103; and 5,201,958. Suchsystems provide additional background to the invention and are,accordingly, incorporated herein by reference.

In one aspect, the invention provides a system for delivering broadbandultrasound to liquid. The system includes first and second ultrasonictransducers. The first transducer has a first frequency and a firstultrasound bandwidth, and the second transducer has a second frequencyand a second ultrasound bandwidth. The first and second bandwidths areoverlapping with each other and the first frequency is different fromthe second frequency. An ultrasound generator drives the transducers atfrequencies within the bandwidths. Together, the first and secondtransducers and the generator produce ultrasound within the liquid andwith a combined bandwidth that is greater than either of the first andsecond bandwidths.

In another aspect, the system of the invention includes a thirdultrasonic transducer that has a third frequency and a third ultrasoundbandwidth. The third bandwidth is overlapping with at least one of theother bandwidths, and the third frequency is different from the firstand second frequencies. The generator in this aspect drives the thirdtransducer within the third bandwidth so as to produce ultrasound withinthe liquid and with a combined bandwidth that is greater than any of thefirst, second and third bandwidths.

Preferably, each of the transducers are clamped so as to resist materialstrain and fatigue. In another aspect, each of the first and secondfrequencies are harmonic frequencies of the transducer's base resonantfrequency. In one aspect, these harmonic frequencies are between about100 khz and 350 khz.

In another aspect, the system includes at least one other synergisticultrasonic transducer that has a synergistic frequency and a synergisticultrasound bandwidth. As above, the synergistic bandwidth is overlappingwith at least one of the other bandwidths, and the synergistic frequencyis different from the first and second frequencies. The generator drivesthe synergistic transducer within the synergistic bandwidth so as toproduce ultrasound within the liquid and with a combined bandwidth thatis greater than any of the other bandwidths. In one aspect, thissynergistic frequency is a harmonic frequency between about 100 khz and350 khz.

In other aspects, the bandwidths of combined transducers overlap sothat, in combination, the transducers produce ultrasonic energy atsubstantially all frequencies within the combined bandwidth. Preferably,the combined operation provides ultrasound with relatively equal powerfor any frequency in the combined bandwidth. Using the full width halfmaximum (FWHM) to define each bandwidth, the bandwidths preferablyoverlap such that the power at each frequency within the combinedbandwidth is within a factor of two of ultrasonic energy produced at anyother frequency within the combined bandwidth.

In another aspect, a system is provided for delivering ultrasound toliquid. The system has an ultrasonic transducer with a harmonicfrequency between about 100 khz and 350 khz and within an ultrasoundbandwidth. A clamp applies compression to the transducer. An ultrasoundgenerator drives the transducer at a range of frequencies within thebandwidth so as to produce ultrasound within the liquid.

In still another aspect, the system can include at least one otherultrasonic transducer that has a second harmonic frequency within asecond bandwidth. As above, the second frequency is between about 100khz and 350 khz, and the second bandwidth is overlapping, in frequency,with the ultrasound bandwidth. The generator drives the transducers atfrequencies within the bandwidths so as to produce ultrasound within theliquid and with a combined bandwidth that is greater than the bandwidthof a single transducer.

Another aspect of the invention provides a system for deliveringultrasound to liquid. In such a system, one or more ultrasonictransducers have an operating frequency within an ultrasound bandwidth.An ultrasound generator drives the transducers at frequencies within thebandwidth, and also changes the sweep rate of the frequency continuouslyso as to produce non-resonating ultrasound within the liquid.

Preferably, the generator of the invention changes the sweep ratefrequency in one of several ways. In one aspect, for example, the sweeprate is varied as a function of time. In another aspect, the sweep rateis changed randomly. Typically, the sweep rate frequency is changedthrough a range of frequencies that are between about 10-50% of theoptimum sweep rate frequency. The optimum sweep rate frequency isusually between about 1 hz and 1.2 khz; and, therefore, the range offrequencies through which the sweep rate is varied can changedramatically. By way of example, if the optimum sweep rate is 500 hz,then the range of sweep rate frequencies is between about 400 hz and 600hz; and the invention operates by varying the sweep rate within thisrange linearly, randomly, or as a function of time, so as to optimizeprocessing characteristics within the liquid.

The invention further provides a system for delivering ultrasound toliquid. This system includes one or more ultrasonic transducers, eachhaving an operating frequency within an ultrasound bandwidth. Anamplitude modulated ultrasound generator drives the transducers atfrequencies within the bandwidth. A generator subsystem also changes themodulation frequency of the AM, continually, so as to produce ultrasoundwithin the liquid to prevent low frequency resonances at the AMfrequency.

Preferably, the subsystem sweeps the AM frequency at a sweep ratebetween about 1 hz and 100 hz. For extremely sensitive parts and/or tankchemistries, the invention can further sweep the AM sweep rate as afunction of time so as to eliminate possible resonances which might begenerated by the AM sweep rate frequency. This sweeping of the AM sweeprate occurs for a range of AM sweep frequencies generally defined by10-40% of the optimum AM sweep rate. For example, if the optimum AMsweep rate is 150 hz, then one aspect of the invention changes the AMsweep rate through a range of about 130 hz to 170 hz.

In one aspect, the invention also provides amplitude control through thepower lines. Specifically, amplitude modulation is achieved by selectinga portion of a leading quarter sinusoid, in a full wave amplitudemodulation pattern, that ends at a selected amplitude in a regionbetween zero and 90° and between 180° and 270° of the sinusoid.Alternatively, amplitude control is achieved by selecting a portion of aleading quarter sinusoid, in a half wave amplitude modulation pattern,that ends at a selected amplitude between zero and 90° of the sinusoid.

In still another aspect, a system of the invention can include two ormore ultrasound generators that are synchronized in magnitude and phaseso that there is substantially zero frequency difference between signalsgenerated by the generators. Preferably, a timing signal is generatedbetween the generators to synchronize the signals. In one aspect, a FMgenerator provides a master frequency modulated signal to each generatorto synchronize the signals from the generators.

A generator of the invention can also be frequency modulated over arange of frequencies within the bandwidth of each transducer. In anotheraspect, the frequency modulation occurs over a range of frequencieswithin the bandwidth of each transducer, and the generator is amplitudemodulated over a range of frequencies within the bandwidth of eachtransducer.

The systems of the invention generally include a chamber for holding thesolution or liquid which is used to clean or process objects therein.The chamber can include, for example, material such as 316L stainlesssteel, 304 stainless steel, polytetrafluoroethylene, fluorinatedethylene propylene, polyvinylidine fluoride, perfluoroalkoxy,polypropylene, polyetheretherketone, tantalum, teflon coated stainlesssteel, titanium, hastalloy, and mixtures thereof.

It is preferable that the transducers of the system include an array ofultrasound transducer elements.

The invention also provides a method of delivering broadband ultrasoundto liquid, including the steps of: driving a first ultrasound transducerwith a generator at a first frequency and within a first ultrasoundbandwidth, and driving a second ultrasound transducer with the generatorat a second frequency within a second ultrasound bandwidth that overlapsat least part of the first bandwidth, such that the first and secondtransducers, in combination with the generator, produce ultrasoundwithin the liquid and with a combined bandwidth that is greater thaneither of the first and second bandwidths.

In other aspects, the method includes the step of compressing at leastone of the transducers, and/or the step of driving the first and secondtransducers at harmonic frequencies between about 100 khz and 350 khz.

Preferably, the method includes the step of arranging the bandwidths tooverlap so that the transducers and generator produce ultrasonic energy,at each frequency, that is within a factor of two of ultrasonic energyproduced by the transducers and generator at any other frequency withinthe combined bandwidth.

The application of broadband ultrasound has certain advantages. First,it increases the useful bandwidth of multiple transducer assemblies sothat the advantages to sweeping ultrasound are enhanced. The broadbandultrasound also gives more ultrasonic intensity for a given power levelbecause there are additional and different frequencies spaced furtherapart in the ultrasonic bath at any one time. Therefore, there is lesssound energy cancellation because only frequencies of the samewavelength, the same amplitude and opposite phase cancel effectively.

In one aspect, the method of the invention includes the step of drivingan ultrasonic transducer in a first bandwidth of harmonic frequenciescentered about a microsonic frequency in the range of 100 khz and 350khz. For protection, the transducer is preferably compressed to protectits integrity.

Another method of the invention provides the following steps: couplingone or more ultrasonic transducers to the liquid, driving, with agenerator, the transducers to an operating frequency within anultrasound bandwidth, the transducers and generator generatingultrasound within the liquid, changing the frequency within thebandwidth at a sweep rate, and continuously varying the sweep rate as afunction of time so as to reduce low frequency resonances.

In other aspects, the sweep rate is varied according to one of thefollowing steps: sweeping the sweep rate as a function of time; linearlysweeping the sweep rate as a function of time; and randomly sweeping thesweep rate. Usually, the optimum sweep frequency is between about 1 hzand 1.2 khz, and therefore, in other aspects, the methods of theinvention change the sweep rate within a range of sweep frequenciescentered about an optimum sweep frequency. Typically, this range isdefined by about 10-50% of the optimum sweep frequency. For example, ifthe optimum sweep frequency is 800 hz, then the range of sweepfrequencies is between about 720 hz and 880 hz. Further, and in anotheraspect, the rate at which the invention sweeps the sweep rate withinthis range is varied at less than about {fraction (1/10)}th of theoptimum frequency. Therefore, in this example, the invention changes thesweep rate at a rate that is less than about 80 hz.

Another method of the invention provides for the steps of (a) generatinga drive signal for one or more ultrasonic transducers, each having anoperating frequency within an ultrasound bandwidth, (b) amplitudemodulating the drive signal at a modulation frequency, and (c) sweepingthe modulation frequency, selectively, as to produce ultrasound withinthe liquid.

The invention is particularly useful as an ultrasonic system whichcouples acoustic energy into a liquid for purposes of cleaning parts,developing photosensitive polymers, and stripping material fromsurfaces. The invention can provide many sound frequencies to the liquidby sweeping the sound through the bandwidth of the transducers. Thisprovides at least three advantages: the standing waves causingcavitation hot spots in the liquid are reduced or eliminated; partresonances within the liquid at ultrasonic frequencies are reduced oreliminated; and the ultrasonic activity in the liquid builds up to ahigher intensity because there is less cancellation of sound waves.

In one aspect, the invention provides an ultrasonic bath withtransducers having at least two different resonant frequencies. In oneconfiguration, the resonant frequencies are made so that the bandwidthsof the transducers overlap and so that the impedance versus frequencycurve for the paralleled transducers exhibit maximum flatness in theresonant region. For example, when a 40 khz transducer with a 4.1 khzbandwidth is put in parallel—i.e., with overlapping bandwidths—with a 44khz transducer with a 4.2 khz bandwidth, the resultant bandwidth of themultiple transducer assembly is about 8 khz. If transducers with threedifferent frequencies are used, the bandwidth is approximately threetimes the bandwidth of a single transducer.

In another aspect, a clamped transducer array is provided with aresonant frequency of 40 khz and a bandwidth of 4 khz. The array has asecond harmonic resonant frequency at 104 khz with a 4 khz harmonicbandwidth. Preferably, the bandwidth of this second harmonic frequencyresonance is increased by the methods described above for thefundamental frequency of a clamped transducer array.

In one aspect, the invention provides a method and associated circuitrywhich constantly changes the sweep rate of an ultrasonic transducerwithin a range of values that is in an optimum process range. Forexample, one exemplary process can have an optimum sweep rate in therange 380 hz to 530 hz. In accord with one aspect of the invention, thissweep rate constantly changes within the 380 hz to 530 hz range so thatthe sweep rate does not set up resonances within the tank and set up aresonance at that rate.

The invention provides for several methods to change the sweep rate. Oneof the most effective methods is to generate a random change in sweeprate within the specified range. A simpler method is to sweep the sweeprate at some given function of time, e.g., linearly. One problem withsweeping the sweep rate is that the sweeping function of time has aspecific frequency which may itself cause a resonance. Accordingly, oneaspect of the invention is to sweep this time function; however, inpractice, the time function has a specific frequency lower than thelowest resonant frequency of the semiconductor wafer or delicate part,so there is little need to eliminate that specific frequency.

Most prior art ultrasonic systems are amplitude modulated at a lowfrequency, typically 50 hz, 60 hz, 100 hz, or 120 hz. One ultrasonicgenerator, the proSONIK™ sold by Ney Ultrasonics Inc., and producedaccording to U.S. Pat. No. 4,736,130, permits the generation of aspecific amplitude modulation pattern that is typically between 50 hz to5 khz. However, the specific amplitude modulation frequency can itselfbe a cause of low frequency resonance in an ultrasonic bath if theselected amplitude modulation frequency is a resonant frequency of thedelicate part.

Accordingly, one aspect of the invention solves the problem of delicatepart resonance at the amplitude modulation frequency by randomlychanging or sweeping the frequency of the amplitude modulation within abandwidth of amplitude modulation frequencies that satisfy the processspecifications. For cases where substantially all of the low frequenciesmust be eliminated, random changes of the modulation frequency arepreferred. For cases where there are no resonances in a part below aspecified frequency, the amplitude modulation frequency can be swept ata frequency below the specified frequency.

Random changing or sweeping of the amplitude modulation frequencyinhibits low frequency resonances because there is little repetitiveenergy at a frequency within the resonant range of the delicate part orsemiconductor wafer. Accordingly, a resonant condition does not buildup, in accord with the invention, providing obvious advantages.

The invention also provides relatively inexpensive amplitude control ascompared to the prior art. One aspect of the invention providesamplitude control with a full wave or half wave amplitude modulatedultrasonic signal. For full wave, a section of the 0° to 90° and the180° to 270° quarter sinusoid is chosen which ends at the required(desired) amplitude. For example, at the zero crossover of the halfsinusoid (0° and 180°), a monostable multivibrator is triggered. It isset to time out before 90° duration, and specifically at the requiredamplitude value. This timed monostable multivibrator pulse is used toselect that section of the quarter sinusoid that never exceeds therequired amplitude.

In one aspect, the invention also provides an adjustable ultrasonicgenerator. One aspect of this generator is that the sweep rate frequencyand the amplitude modulation pattern frequency are randomly changed orswept within the optimum range for a selected process. Another aspect isthat the generator drives an expanded bandwidth clamped piezoelectrictransducer array at a harmonic frequency from 100 khz to 350 khz.

Such a generator provides several improvements in the problematic areasaffecting lower frequency ultrasonics and megasonics: uncontrolledcavitation implosion, unwanted resonances, unreliable transducers, andstanding waves. Instead, the system of the invention provides uniformmicrostreaming that is critical to semiconductor wafer and otherdelicate part processing and cleaning.

In another aspect of the invention, an array of transducers is used totransmit sound into a liquid at its fundamental frequency, e.g., 40 khz,and at each harmonic frequency, e.g., 72 khz or 104 khz. The outputs ofgenerators which have the transducer resonant frequencies and harmonicfrequencies are connected through relays to the transducer array. Onegenerator with the output frequency that most closely producers theoptimum energy in each cavitation implosion for the current processchemistry is switched to the transducer array.

In yet another aspect, the invention reduces or eliminates low frequencybeat resonances created by multiple generators by synchronizing thesweep rates (both in magnitude and in phase) so that there is zerofrequency difference between the signals coming out of multiplegenerators. In one aspect, the synchronization of sweep rate magnitudeand phase is accomplished by sending a timing signal from one generatorto each of the other generators. In another aspect, a master FM signalis generated that is sent to each “slave” power module, which amplifiesthe master FM signal for delivery to the transducers. At times, themaster and slave aspect of the invention also provides advantages ineliminating or reducing the beat frequency created by multiplegenerators driving a single tank.

However, when multiple generators are driving different tanks in thesame system, this master and slave aspect may not be acceptable becausethe AM of the FM signal is usually different for different processes inthe different tanks. Accordingly, and in another aspect, a mastercontrol is provided which solves this problem. The master control of theinvention has a single FM function generator (sweeping frequency signal)and multiple AM function generators, one for each tank. Thus, every tankin the system receives the same magnitude and phase of sweep rate, but adifferent AM as set on the control for each generator.

The invention also provides other advantages as compared to the priorart's methods for frequency sweeping ultrasound within the transducer'sbandwidth. Specifically, the invention provides a sweeping of the sweeprate, within the transducer's bandwidth, such that low frequencyresonances are reduced or eliminated. Prior art frequency sweep systemshad a fixed sweep frequency that is selectable, once, for a givenapplication. One problem with such prior art systems is that the singlelow frequency can set up a resonance in a delicate part, for example, aread-write head for a hard disk drive.

The invention also provides advantages in that the sweep frequency ofthe sweep rate can be adjusted to conditions within the tank, or to theconfiguration of the tank or transducer, or even to a process chemistry.

The invention also has certain advantages over prior art single chamberultrasound systems. Specifically, the methods of the invention, incertain aspects, use different frequency ultrasonics for each differentchemistry so that the same optimum energy in each cavitation implosionis maintained in each process or cleaning chemistry. According to otheraspects of the invention, this process is enhanced by selecting theproper ultrasonic generator frequency that is supplied at thefundamental or harmonic frequency of the transducers bonded to thesingle ultrasonic chamber.

These and other aspects and advantages of the invention are evident inthe description which follows and in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained byreference to the drawings, in which:

FIG. 1 shows a cut-away side view schematic of an ultrasound processingsystem constructed according to the invention;

FIG. 1A shows a top view schematic of the system of FIG. 1;

FIG. 2 shows a schematic illustration of a multi-transducer systemconstructed according to the invention and used to generate broadbandultrasound in a combined bandwidth;

FIG. 2A graphically illustrates the acoustic disturbances produced bythe two transducers of FIG. 2;

FIG. 2B graphically illustrates the broadband acoustic disturbancesproduced by harmonics of a multi-transducer system constructed accordingto the invention;

FIG. 3 shows a block diagram illustrating one embodiment of a systemconstructed according to the invention;

FIG. 4 shows a schematic embodiment of the signal section of the systemof FIG. 3;

FIGS. 5A and 5B show a schematic embodiment of the power module sectionof the system FIG. 3;

FIG. 6 is a cross-sectional side view of a harmonic transducerconstructed according to the invention and driven by the power module ofFIGS. 5A and 5B; FIG. 6A is a top view of the harmonic transducer ofFIG. 6;

FIG. 7 is a schematic illustration of an amplitude control subsystemconstructed according to the invention;

FIG. 7A shows illustrative amplitude control signals generated by anamplitude control subsystem such as in FIG. 7;

FIG. 8 shows a schematic illustration of an AM sweep subsystemconstructed according to the invention;

FIG. 8A shows a typical AM frequency generated by an AM generator,

FIG. 8B graphically shows AM sweep frequency as a function of time for arepresentative sweep rate, in accord with the invention;

FIG. 9 illustrates a multi-generator, multi-frequency, single tankultrasound system constructed according to the invention;

FIG. 9A illustrates another multi-generator, single tank systemconstructed according to the invention;

FIG. 10 illustrates a multi-generator, common-frequency, single tankultrasound system constructed according to the invention;

FIG. 11 illustrates a multi-tank ultrasound system constructed accordingto the invention;

FIG. 11A shows representative AM waveform patterns as controlled throughthe system of FIG. 11; and

FIGS. 12A, 12B and 12C graphically illustrate methods of sweeping thesweep rate in accord with the invention.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

FIGS. 1 and 1A show schematic side and top views, respectively, of anultrasound processing system 10 constructed according to the invention.An ultrasonic generator 12 electrically connects, via electrical paths14 a, 14 b, to an ultrasound transducer 16 to drive the transducer 16 atultrasound frequencies above about 18 khz, and usually between 40 khzand 350 khz. Though not required, the transducer 16 is shown in FIG. 1as an array of transducer elements 18. Typically, such elements 18 aremade from ceramic, piezoelectric, or magnetostrictive materialswhich.expand and contract with applied voltages or current to createultrasound. The transducer 16 is mounted to the bottom, to the sides, orwithin the ultrasound treatment tank 20 through conventional methods,such as known to those skilled in the art and as described above. Aliquid 22 fills the tank to a level sufficient to cover the delicatepart 24 to be processed and/or cleaned. In operation, the generator 12drives the transducer 16 to create acoustic energy 26 that couples intothe liquid 22.

Although the transducer 16 is shown mounted to the bottom of the tank20, those skilled in the art will appreciate that other mountingconfigurations are possible and envisioned. The transducer elements 18are of conventional design, and are preferably “clamped” so as tocompress the piezoelectric transducer material.

FIG. 2 illustrates a two transducer system 30. Transducer 32 a, 32 b aresimilar to one of the elements 18, FIG. 1. Transducer 32 a includes twoceramic sandwiched elements 34, a steel back plate 38 a, and a frontdrive plate 36 a that is mounted to the tank 20′. Transducer 32 bincludes two ceramic sandwiched elements 34, a steel back plate 38 b,and a front drive plate 36 b that is mounted to the tank 20′. Bolts 39a, 39 b pass through the plates 38 a, 38 b and screw into the driveplates 36 a, 36 b, respectively, to compresses the ceramics 34. Thetransducers 32 are illustratively shown mounted to a tank surface 20′.

The transducers 32 a, 32 b are driven by a common generator such asgenerator 12 of FIG. 1. Alternatively, multiple generators can be used.The ceramics 34 are oriented with positive “+” orientations together orminus “−” orientations together to obtain cooperative expansion andcontraction within each transducer 32. Lead-outs 42 illustrate theelectrical connections which connect between the generator and thetransducers 32 so as to apply a differential voltage there-across. Thebolts 39 a, 39 b provide a conduction path between the bottoms 43 andtops 45 of the transducers 32 to connect the similar electrodes (hereshown as −, −) of the elements 34.

The thicknesses 40 a, 40 b of transducers 32 a, 32 b, respectively,determine the transducer's fundamental resonant frequency. For purposesof illustration, transducer 32 a has a fundamental frequency of 40 khz,and transducer 32 b has a fundamental frequency of 44 khz. Transducers32 a, 32 b each have a finite ultrasound bandwidth which can beadjusted, slightly, by those skilled in the art. Typically, however, thebandwidths are about 4 khz. By choosing the correct fundamentalfrequencies, therefore, an overlap between the bandwidths of the twotransducers 32 a, 32 b can occur, thereby adding additional range withinwhich to apply ultrasound 26 a′, 26 b′ to liquid 22′.

The acoustic energy 26′ applied to the liquid 22′ by the combination oftransducers 32 a, 32 b is illustrated graphically in FIG. 2A. In FIG.2A, the “x” axis represents frequency, and the “y” axis representsacoustical power. The outline 44 represents the bandwidth of transducer32 a, and outline 46 represents the bandwidth of transducer 32 b.Together, they produce a combined bandwidth 43 which produces arelatively flat acoustical energy profile to the liquid 22′, such asillustrated by profile 48. The flatness of the bandwidth 43 representingthe acoustical profile 48 of the two transducers 32 a, 32 b ispreferably within a factor of two of any other acoustical strengthwithin the combined bandwidth 43. That is, if the FWHM defines thebandwidth 43; the non-uniformity in the profile 48 across the bandwidth43 is typically better than this amount. In certain cases, the profile48 between the two bandwidths 44 and 46 is substantially flat, such asillustrated in FIG. 2A.

The generator connected to lead-outs 42 drives the transducers 32 a, 32b at frequencies within the bandwidth 43 to obtain broadband acousticaldisturbances within the liquid 22′. As described herein, the manner inwhich these frequencies are varied to obtain the overall disturbance isimportant. Most preferably, the generator sweeps the frequencies throughthe overall bandwidth, and at the same time sweeps the rate at whichthose frequencies are changed. That is, one preferred generator of theinvention has a “sweep rate” that sweeps through the frequencies withinthe bandwidth 43; and that sweep rate is itself varied as a function oftime. In alternative embodiments of the invention, the sweep rate isvaried linearly, randomly, and as some other function of time tooptimize the process conditions within the tank 20′.

With further reference to FIGS. 1 and 1A, each of the elements 18 canhave a representative bandwidth such as illustrated in FIG. 2A.Accordingly, an even larger bandwidth 43 can be created with three ormore transducers such as illustrated by transducers 32 a, 32 b. Inparticular, any number of combined transducers can be used. Preferably,the bandwidths of all the combined transducers overlap to provide anintegrated bandwidth such as profile 48 of FIG. 2A. As such, eachtransducer making up the combined bandwidth should have a uniqueresonant frequency.

Those skilled in the art understand that each of the transducers 18 and32 a, 32 b, FIGS. 1 and 2A, respectively, have harmonic frequencieswhich occur at higher mechanical resonances of the primary resonantfrequency. It is one preferred embodiment of the invention that suchtransducers operate at one of these harmonics, i.e., typically thefirst, second, third or fourth harmonic, so as to function in thefrequency range of 100 khz to 350 khz. This frequency range provides amore favorable environment for acoustic processes within the tanks 20,20′ as compared to low frequency disturbances less than 100 khz. Forexample, ultrasound frequencies around the 40 khz frequency can easilycause cavitation damage in the part 24. Further, such frequencies tendto create standing waves and other hot spots of spatial cavitationwithin the liquid.

Accordingly, the benefits of applying a broadband acoustic disturbanceto the liquid also apply to the 100-350 khz microsonic frequencies.Similar to FIG. 2A, FIG. 2B illustrates a combined bandwidth 50 ofharmonic frequencies in the range 100-350 khz. Specifically, FIG. 2Bshows the combined bandwidth 50 that is formed by the bandwidth 44′around the second harmonic of the 40 Khz frequency, and the bandwidth46′ around the second harmonic of the 41.5 khz frequency.

FIG. 3 shows in block diagram embodiment of a system 110 constructedaccording to the present invention. The system 110 includes a signalsection 112 which drives a power module 121. The power module 121 powersthe harmonic transducer array 122. The transducer array 122 are coupledto a liquid 123 by one of several conventional means so as to generateacoustic energy within the liquid 123. By way of example, the array 122is similar to the array 16 of FIG. 1; and the liquid 123 is similar tothe liquid 22 of FIG. 1.

The signal section 112 includes a triangle wave oscillator 114 with afrequency typically below 150 hz. The purpose of the oscillator 114 isto provide a signal that sweeps the sweep rate of the ultrasoundfrequencies generated by the transducer arrays 122.

The oscillator 114 is fed into the input of the sweep rate VCO 115(Voltage Controlled Oscillator). This causes the frequency of the outputof VCO 115 to linearly sweep at the frequency of the oscillator 114. Theoptimum sweep rate frequency output of VCO 115 is typically from about10 hz, for magnetostrictive elements, to about 1.2 khz, forpiezoelectrics. Therefore, the optimum center sweep rate frequency canbe anywhere within the range of about 10 hz to 1.2 khz, and that sweeprate is varied within a finite range of frequencies about the centersweep frequency. This finite range is typically set to about 10-50% ofthe center sweep rate frequency. For example, the center sweep ratefrequency for one process might be 455 hz, so the VCO 115 output is set,for example, to sweep from 380 hz to 530 hz. If, additionally, theoscillator 114 is set to 37 hz, then the output of VCO 115 changesfrequency, linearly, from 380 hz to 530 hz, and back to 380 hz at thirtyseven times per second.

The output of VCO 115 feeds the VCO input of the 2×center frequency VCO116. The VCO 116 operates as follows. If, for example, the centerfrequency of VCO 116 is set to 208 khz and the bandwidth is set to 8khz, the center frequency linearly changes from 204 khz to 212 khz andback to 204 khz in a time of 1.9 milliseconds (i.e., {fraction (1/530)}hz) to 2.63 milliseconds (i.e., {fraction (1/380)} hz). The specifictime is determined by the voltage output of the oscillator 114 at thetime of measurement. Since the voltage output of oscillator 114 isconstantly changing, the time it takes to linearly sweep the centerfrequency from 204 khz to 212 khz and back to 204 khz is also constantlychanging. In this example, the time changes linearly from 1.9 ms to 2.63ms and back to 1.9 ms at thirty seven times per second.

The oscillator 114, VCO 115 and VCO 116 operate, in combination, toeliminate the repetition of a single sweep rate frequency in the rangeof 10 hz to 1.2 khz. For example, the highest single frequency thatexists in the stated example system is 37 hz. If an unusual applicationor process were found whereby a very low frequency resonance around 37hz exists, then the oscillator 114 would be replaced by a random voltagegenerator to reduce the liklihood of exciting any modes within the part.

The VCO 116 drives a divide-by-two D flip-flop 117. The purpose of the Dflip-flop 117 is to eliminate asymmetries in the waveform from the VCO116. The output of the D flip-flop 117 is thus a square wave that hasthe desired frequency which changes at a sweep rate that is itselfsweeping. In the stated example, the output square wave from D flip-flop117 linearly changes from 102 khz to 106 khz and back to 102 khz atdifferent times in the range of 1.9 ms to 2.63 ms. This sweeping of thesweep rate is sometimes referred to herein as “double sweep” or “doublesweeping.”

The AC line zero-crossover detection circuit 118 produces a signal witha rise time or narrow pulse at or near the time that the AC line voltageis at zero or at a low voltage, i.e., at or near zero degrees. Thissignal triggers the adjustable monstable multivibrator 119. The timedpulse out of monostable multivibrator 119 is set to a value between zerodegrees and ninety degrees, which corresponds to a time from zero to4.17 ms for a 60 hz line frequency.

If the maximum amplitude were desired, for example, the monostablemultivibrator 119 is set to a time of 4.17 ms for a 60 hz linefrequency. For an amplitude that is 50% of maximum, the monostablemultivibrator 119 is set to 1.389 ms for a 60 hz line frequency. Ingeneral, the monostable multivibrator 119 time is set to the arcsin ofthe amplitude percent times the period of the line frequency divided by360 degrees.

The double sweeping square wave output of the D flip-flop 117 and thetimed pulse output of the monostable multivibrator 119 feed into thesynchronization logic 120. The synchronization logic 120 performs threeprimary functions. First, it only allows the double sweeping square waveto pass to the output of the synchronization logic 120 during the timedefined by the pulse from the monostable multivibrator 119. Second, thesynchronization logic 120 always allows a double sweeping square wavewhich starts to be completed, even if the monostable multivibrator 119times out in the middle of a double sweeping square wave. And lastly,the synchronization logic 120 always starts a double sweeping squarewave at the beginning of the ultrasonic frequency, i.e., at zerodegrees.

The output of synchronization logic 120 is a double sweeping square wavethat exists only during the time defined by the monostable multivibrator119 or for a fraction of a cycle past the end of the monostablemultivibrator 119 time period. The synchronization logic 120 outputfeeds a power module 121 which amplifies the pulsed double sweepingsquare wave to an appropriate power level to drive the harmonictransducers 122. The transducers 122 are typically bonded to a tank anddeliver sound waves into the liquid within the tank. These sound wavesduplicate the pulsed double sweeping characteristics of the output ofthe signal section 112.

FIG. 4 shows a schematic embodiment of the signal section 112 in FIG. 3.U1 is a XR-2209 precision oscillator with a triangle wave output at pin8. The frequency of the XR-2209 is 1/(RC)=1/((27 k) (1 μf))=37 hz. Thissets the frequency of the triangle wave oscillator 114, FIG. 3, to sweepthe sweep rate at 37hz. The other components associated with the XR-2209are the standard configuration for single supply operation of thisintegrated circuit.

U2 is a XR-2209 precision oscillator with a triangle wave output atpin8. The center frequency of U2 is 1/(RC)=1/((2.2 k) (1 μf))=455 hz.The actual output frequency is proportional to the current flowing outof pin4 of U2. At 455 hz, this current is 6 volts/2.2 k=2.73 ma. It isgenerally desirable, according to the invention, to sweep the 455 hzsweep rate through a total change of 150 hz, i.e., 75 hz either side of455 hz. Since 75 hz/455 hz=16.5%, the current flowing out of pin 4 mustchange by 16.5% in each direction, that is, by (16.5%) (2.73 ma)=0.45ma. The triangle wave from U1 causes this change. The triangle wavechanges from 3 volts to 9 volts; therefore, there is 3 volts on eitherside of 6 volts at pin4 of U2 to cause the 0.45 ma change. By makingR1=3 volts/0.45 ma=6.67 kΩ, the sweep rate is changed 75 hz either sideof 455 hz. The actual R1 used in FIG. 4 is 6.65 kΩ, a commerciallyavailable value giving an actual change of 75.2 hz.

U3 is a XR-2209 precision oscillator with a center frequency ofapproximately 1/(RC)=1/((12 k+2.5 k) (330 pf))=209 khz with thepotentiometer set to its center position of 2.5 kΩ. In the actualcircuit, the potentiometer is adjusted to about 100Ω higher to give thedesired 208 khz center frequency. Out of U3 pin4 flows 6 volts/(12kΩ+2.5 kΩ+100Ω)=0.41 ma. To change the center frequency a total of 8khz, the 0.41 ma is changed by 4 khz/208 khz=1.92%, or 7.88 μa. Thismeans that R2=3 volts/7.88 μa=381 kΩ. In FIG. 4, however, the commercialvalue of 383 kΩ was used.

U3 pin7 has a square wave output that is changing from 204 khz to 212khz and back to 204 khz at a rate between 380 hz and 530 hz. The actualrate is constantly changing thirty 23 seven times a second as determinedby U1.

U4 is a D flip-flop in a standard divide by two configuration. Itsquares up any non 50% duty cycle from U3 and provides a frequency rangeof 102 khz to 106 khz from the 204 khz to 212 khz U3 signal.

The output of U4 feeds the synchronization logic which is describedbelow and after the description of the generation of the amplitudecontrol signal.

The two 1N4002 diodes in conjunction with the bridge rectifier form afull wave half sinusoid signal at the input to the 40106 Schmidt triggerinverter. This invertor triggers when the half sinusoid reaches about 7volts, which on a half sinusoid with an amplitude of 16 times the squareroot of two is close enough to the zero crossover for a trigger point ina practical circuit. The output of the 40106 Schmidt trigger falls whichtriggers U5, the edge triggered 4538 monostable multivibrator wired in atrailing edge trigger/retriggerable configuration. The output of U5 goeshigh for a period determined by the setting on the 500 kΩ potentiometer.At the end of this period, the output of U5 goes low. The period ischosen by setting the 500 kΩ potentiometer to select that portion of theleading one-quarter sinusoid that ends at the required amplitude to giveamplitude control. This timed positive pulse feeds into thesynchronization logic along with the square wave output of U4.

The timed pulse U5 feeds the D input of U6, a 4013 D-type flip flop. Thesquare wave from U4 is invented by U7 a and feeds the clock input of U6.U6 only transfers the signal on the D input to the output Q at the riseof a pulse on the clock input, Pin3. Therefore, the Q output of U6 onPin1 is high when the D input of U6 on Pin3 is high and the clock inputof U6 on Pin3 transitions high. This change in the Q output of U6 istherefore synchronized with the change in the square wave from U4.

The synchronized high Q output of U6 feeds U8 Pin13, a 4093 Schmidttrigger NAND gate. The high level on Pin13 of U8 allows the square wavesignal to pass from U8 Pin12 to the output of U8 at Pin11.

In a similar way, U8 synchronizes the falling output from U5 with thesquare wave from U4. Therefore, only complete square waves pass to U8Pin11 and only during the time period as chosen by monostablemultivibrator U5. The 4049 buffer driver U7 b inverts the output at U8Pin11 so it has the same phase as the square wave output from U4. Thissignal, U7b Pin2 is now the proper signal to be amplified to drive thetransducers.

FIGS. 5A, 5B represent a circuit that increases the signal from U7b Pin2 in FIG. 4 to a power level for driving the transducers 122, FIG. 3.There are three isolated power supplies. The first one, including a T1i,a bridge, C19, VR1 and C22, produces +12 VDC for the input logic. Thesecond and third isolated power supplies produce +15 VDC at VR2 Pin3 andVR3 Pin3 for gate drive to the IGBT's (insulated gate bipolartransistors).

The signal input to FIGS. 5A, SB has its edges sharpened by the 40106Schmidt trigger U9 a. The output of U9 a feeds the 4049 buffer driversU10 c and U10 d which drive optical isolator and IGBT driver U12, aHewlett Packard HCPL3120. Also, the output of U9 a is inverted by U9 band feeds buffer drivers U10 a and U10 b which drive U11, anotherHCPL3120.

This results in an isolated drive signal on the output of U11 and thesame signal on the output of U12, only 180° out of phase. Therefore, U11drives Q1 on while U12 drives Q2 off. In this condition, a power halfsinusoid of current flows from the high voltage full wave DC at B1through D1 and Q1 and L1 into C1. Current cannot reverse because it isblocked by D1 and the off Q2. When the input signal changes state, U11turns off Q1 and U12 turns on Q2, a half sinusoid of current flow out ofC1 through L2 and D2 and Q2 back into C1 in the opposite polarity. Thisends a complete cycle.

The power signal across C1 couples through the high frequency isolationtransformer T4. The output of T4 is connected to the transducer ortransducer array.

FIG. 6 shows a cross-sectional side view of one clamped microsonictransducer 128 constructed according to the invention; while FIG. 6Ashows a top view of the microsonic transducer 128. The microsonictransducer 128 has a second harmonic resonant frequency of 104 khz witha 4 khz bandwidth (i.e., from 102 khz to 106 khz). The cone-shapedbackplate 139 flattens the impedance verses frequency curve to broadenthe frequency bandwidth of the microsonic transducer 128. Specifically,the backplate thickness along the “T” direction changes fortranslational positions along direction “X.” Since the harmonicresonance of the microsonic transducer 128 changes as a function ofbackplate thickness, the conical plate 139 broadens and flattens themicrosonic transducer's operational bandwidth.

The ceramic 134 of microsonic transducer 128 is driven throughoscillatory voltages transmitted across the electrodes 136. Theelectrodes 136 connect to an ultrasonic generator (not shown), such asdescribed above, by insulated electrical connections 138. The ceramic134 is held under compression through operation of the bolt 132.Specifically, the bolt 132 provides 5,000 pounds of compressive force onthe piezoelectric ceramic 134.

Amplitude control according to one embodiment of the invention isillustrated in FIGS. 7 and 7A. Specifically, FIG. 7 shows an amplitudecontrol subsystem 140 that provides amplitude control by selecting aportion of the rectified line voltage 145 which drives the ultrasonicgenerator amplitude select section 146. The signal section 112, FIG. 3,and particularly the monostable multivibrator 119 and synchronizationlogic 120, provide similar functionality. In FIG. 7, the amplitudecontrol subsystem 140 operates with the ultrasonic generator 142 andconnects with the power line voltage 138. The rectification section 144changes the ac to dc so as to provide the rectified signal 145.

The amplitude select section 146 selects a portion of the leadingquarter sinusoid of rectified signal 145 that ends at the desiredamplitude, here shown as amplitude “A,” in a region 148 between zero and90° and in a region 150 between 180° and 270° of the signal 145. In thismanner, the amplitude modulation 152 is selectable in a controlledmanner as applied to the signal 154 driving the transducers 156 from thegenerator 142, such as discussed in connection with FIGS. 3 and 4.

FIG. 7A shows illustrative selections of amplitude control in accordwith the invention. The AC line 158 is first converted to a full wavesignal 160 by the rectifier 144. Thereafter, the amplitude selectsection 146 acquires the signal amplitude selectively. For example, byselecting the maximum amplitude of 90° in the first quarter sinusoid,and 270° in the third quarter sinusoid, a maximum amplitude signal 162is provided. Similarly, a one-half amplitude signal 164 is generated bychoosing the 30° and 210° locations of the same sinusoids. By way of afurther example, a one-third amplitude signal 166 is generated bychoosing 19.5° and 199.5°, respectively, of the same sinusoids.

Those skilled in the art will appreciate that the rectification section144 can also be a half-wave rectifier. As such, the signal 145 will onlyhave a response every other one-half cycle. In this case, amplitudecontrol is achieved by selecting a portion of the leading quartersinusoid that ends at a selected amplitude between zero and 90° of thesinusoid.

The ultrasonic generator of the invention is preferably amplitudemodulated. Through AM control, various process characteristics withinthe tank can be optimized. The AM control can be implemented such asdescribed in FIGS. 3,4,7 and 7A, or through other prior art techniquessuch as disclosed in U.S. Pat. No. 4,736,130.

This “sweeping” of the AM frequency is accomplished in a manner that issimilar to ultrasonic generators which sweep the frequency within thebandwidth of an ultrasonic transducer. By way of example, U.S. Pat. No.4,736,130 describes one ultrasonic generator which provides variableselection of the AM frequency through sequential “power burst”generation and “quiet time” during a power train time. In accord withthe invention, the AM frequency is changed to “sweep” the frequency in apattern so as to provide an AM sweep rate pattern.

FIG. 8 illustrates an AM sweep subsystem 170 constructed according tothe invention. The AM sweep subsystem 170 operates as part of, or inconjunction with, the ultrasonic generator 172. The AM generator 174provides an AM signal 175 with a selectable frequency. Theincrement/decrement section 176 commands the AM generator 174 overcommand line 177 to change its frequency over a preselected time periodso as to “sweep” the AM frequency in the output signal 178 which drivesthe transducers 180.

U.S. Pat. No. 4,736,130 describes one AM generator 56, FIG. 1, that issuitable for use as the generator 174 of FIG. 8. By way of example, FIG.8A illustrates one selectable AM frequency signal 182 formed throughsuccessive 500 μs power bursts and 300 μs quiet times to generate a 1.25khz signal (e.g., 1/(300 μs+500 μs)=1.25 khz). If, for example, the AMfrequency is swept at 500 hz about a center frequency of 1.25 khz, suchas shown in FIG. 8, then the frequency is commanded to vary between 1.25khz+250 hz and 1.25 khz−250 hz, such as illustrated in FIG. 8B. FIG. 8Billustrates a graph of AM frequency versus time for this example.

FIG. 9 schematically illustrates a multi-generator, single tank system200 constructed according to the invention. In many instances, it isdesirable to select an ultrasound frequency 201 that most closelyachieves the cavitation implosion energy which cleans, but does notdamage, the delicate part 202. In a single tank system such as in FIG.9, the chemistries within the tank 210 are changed, from time to time,so that the desired or optimum ultrasound frequency changes. Thetransducers and generators of the prior art do not operate or functionat all frequencies, so system 200 has multiple generators 206 andtransducers 208 that provide different frequencies. By way of example,generator 206 a can provide a 40 khz primary resonant frequency; whilegenerator 206 b can provide the first harmonic 72 khz frequency.Generator 206 c can provide, for example, 104 khz microsonic operation.In the illustrated example, therefore, the generators 206 a, 206 b, 206c operate, respectively, at 40 khz, 72 khz, and 104 khz. Each transducer208 responds at each of these frequencies so that, in tandem, thetransducers generate ultrasound 201 at the same frequency to fill thetank 210 with the proper frequency for the particular chemistry.

In addition, each of the generators 206 a-206 c can and do preferablysweep the frequencies about the transducers' bandwidth centered aboutthe frequencies 40 khz, 72 khz, and 104 khz, respectively; and theyfurther sweep the sweep rate within these bandwidths to reduce oreliminate resonances which might occur at the optimum sweep rate.

When the tank 210 is filled with a new chemistry, the operator selectsthe optimum frequency through the mux select section 212. The mux selectsection connects to the analog multiplexer (“mux”) 214 which connects toeach generator 206. Specifically, each generator 206 couples through themux 214 in a switching network that permits only one active signal line216 to the transducers 208. For example, if the operator at mux selectsection 212 chooses microsonic operation to optimize the particularchemistry in the tank 210, generator 206 c is connected through the mux214 and drives each transducer 208 a-208 c to generate microsonicultrasound 201 which fills the tank 210. If, however, generator 206 a isselected, then each of the transducers 208 are driven with 40 khzultrasound.

FIG. 9A illustrates another single tank, multi-generator system 200′constructed according to the invention. Specifically, like in FIG. 9,each of the generators 206′ provides a different frequency. However,each generator 206′ connects to drive unique transducer arrays 208′within the tank 210′. In this manner, for example, generator 206 a′ isselected to generate 40 khz ultrasound 201 a in the tank 210′; generator206 b′ is selected to generate 72 khz ultrasound 201 b in the tank 210′;and generator 206 c is selected to generate 104 khz microsonics 201 c inthe tank 210′. These generator/transducer pairs 206 a′/208 a′, 206b′/208 b′ and 206 c′/208 c′ do not generally operate at the same time;but rather are selected according to the process chemistries and part202′ in the tank 210′.

Those skilled in the art should appreciate that each of the generators206 can be replaced by multiple generators operating at the same orsimilar frequency. This is sometimes needed to provide additional powerto the tank 210 at the desired frequency. Those skilled in the artshould also appreciate that the mux 214 can be designed in several knownmethods, and that techniques to do so abound in the art.

FIG. 10 illustrates a multi-generator, common frequency ultrasoundsystem 230 constructed according to the invention. In FIG. 10, aplurality of generators 232 (232 a-232 c) connect through signal lines234 (234 a-234 c) to drive associated transducers 238 (238 a-238 c) in acommon tank 236. Each of the transducers 238 and generators 232 operateat the same frequency, and are preferably swept through a range offrequencies such as described above so as to reduce or eliminateresonances within the tank 236 (and within the part 242).

In order to eliminate “beating” between ultrasound energies 240 a-240 cof the the several transducers 238 a-238 c and generators 232 a-232 c,the generators 232 are each driven by a common FM signal 250 such asgenerated by the master signal generator 244. The FM signal is coupledto each generator through signal divider 251.

In operation, system 230 permits the coupling of identical frequencies,in magnitude and phase, into the tank 236 by the several transducers238. Accordingly, unwanted beating effects are eliminated. The signal250 is swept with FM control through the desired ultrasound bandwidth ofthe several transducers to eliminate resonances within the tank 236; andthat sweep rate frequency is preferably swept to eliminate any lowfrequency resonances which can result from the primary sweep frequency.

Those skilled in the art should appreciate that system 230 of FIG. 10can additionally include or employ other features such as describedherein, such as AM modulation and sweep, AM control, and broadbandtransducer.

FIG. 11 illustrates a multi-tank system 260 constructed according to theinvention. One or more generators 262 drive each tank 264 (hereillustrated, generators 262 a and 262 b drive tank 264 a; and generators264 c and 264 d drive tank 264 b). Each of the generators 262 connectsto an associated ultrasound transducer 266 a-d so as to produceultrasound 268 a-d in the associated tanks 264 a-b.

The common master signal generator 270 provides a common FM signal 272for each of the generators 262. Thereafter, ultrasound generators 262a-b generate ultrasound 268 a-b that is identical in magnitude andphase, such as described above. Similarly, generators 262 c-d generateultrasound 268 c-d that is identical in magnitude and phase. However,unlike above, the generators 262 each have an AM generator 274 thatfunctions as part of the generator 262 so as to select an optimum AMfrequency within the tanks 264. In addition, the AM generators 274preferably sweep through the AM frequencies so as to eliminateresonances at the AM frequency.

More particularly, generators 274 a-b generate and/or sweep throughidentical frequencies of the AM in tank 264 a; while generators 274 c-dgenerate and/or sweep through identical frequencies of AM in tank 264 b.However, the AM frequency and/or AM sweep of the paired generators 274a-b need not be the same as the AM frequency and/or AM sweep of thepaired generators 274 c-d. Each of the generators 274 operate at thesame carrier frequency as determined by the FM signal 270; however eachpaired generator set 274 a-b and 274 c-d operates independently from theother set so as to create the desired process characteristics within theassociated tank 264.

Accordingly, the system 260 eliminates or prevents undesirablecross-talk or resonances between the two tanks 264 a-b. Since thegenerators within all tanks 264 operate at the same signal frequency270, there is no effective beating between tanks which could upset ordestroy the desired cleaning and/or processing characteristics withinthe tanks 264. As such, the system 260 reduces the likelihood ofcreating damaging resonances within the parts 280 a-b. It is apparent tothose skilled in the art that the FM control 270 can contain the four AMcontrols 274 a-d instead of the illustrated configuration.

FIG. 11A shows two AM patterns 300 a, 300 b that illustrate ultrasounddelivered to multiple tanks such as shown in FIG. 11. For example, AMpattern 300 a represents the ultrasound 268 a of FIG. 11; while AMpattern 300 b represents the ultrasound 268 c of FIG. 11. With a commonFM carrier 302, as provided by the master generator 270, FIG. 11, theultrasound frequencies 302 can be synchronized so as to eliminatebeating between tanks 264 a, 264 b. Further, the separate AM generators274 a and 274 c provide capability so as to select the magnitude of theAM frequency shown by the envelope waveform 306. As illustrated, forexample, waveform 306 a has a different magnitude 308 as compared to themagnitude 310 of waveform 306 b. Further, generators 374 a, 374 c canchange the periods 310 a, 310 b, respectively, of each of the waveforms306 a, 306 b selectively so as to change the AM frequency within eachtank.

FIGS. 12A, 12B and 12C graphically illustrate the methods of sweepingthe sweep rate, in accord with the invention. In particular, FIG. 12Ashows an illustrative condition of a waveform 350 that has a centerfrequency of 40 khz and that is varied about the center frequency so asto “sweep” the frequency as a function of time along the time axis 352.FIG. 12B illustrates FM control of the waveform 354 which has a varyingperiod 356 specified in terms of time. For example, a 42 khz periodoccurs in 23.8 μs while a 40 khz period occurs in 25 μs. The regions 358a, 358 b are shown for ease of illustration and represent, respectively,compressed periods of time within which the system sweeps the waveform354 through many frequencies from 42 khz to 40 khz, and through manyfrequencies from 40 khz to 38 khz.

FIG. 12c graphically shows a triangle pattern 360 which illustrates thevariation of sweep rate frequency along a time axis 362.

The invention thus attains the objects set forth above, among thoseapparent from preceding description. Since certain changes may be madein the above apparatus and methods without departing from the scope ofthe invention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawing be interpreted asillustrative and not in a limiting sense.

It is also to be understood that the following claims are to cover allgeneric and specific features of the invention described herein, and allstatements of the scope of the invention which, as a matter of language,might be said to fall there between.

Having described the invention, what is claimed as new and secured byLetters Patent is:
 1. A system for delivering broadband ultrasound toliquid, comprising: first and second ultrasonic transducers, the firsttransducer having a first frequency and a first ultrasound bandwidth,the second transducer having a second frequency and a second ultrasoundbandwidth, the first and second bandwidths being overlapping with eachother, the first frequency being different from the second frequency;and ultrasound generator means for driving the transducer at frequencieswithin the bandwidths, the first and second transducers and thegenerator means being constructed and arranged so as to produceultrasound within the liquid and with a combined bandwidth that isgreater than either of the first or second bandwidths, wherein eachtransducer comprises an array of ultrasound transducer elements, eachelement within the array being driven at substantially the samefrequency as other elements within the same array.
 2. A system accordingto claim 1, further comprising a third ultrasonic transducer having athird frequency and a third ultrasound bandwidth, the third bandwidthbeing overlapping with at least one of the other bandwidths, the thirdfrequency being different from the first and second frequencies, andwherein the generator means comprises means for driving the thirdtransducer within the third bandwidth so as to produce ultrasound withinthe liquid and with a combined bandwidth that is greater than either ofthe first, second or third bandwidths.
 3. A system according to claim 1,wherein the first frequency is about 40 khz and the first bandwidth isabout 4.1 khz, and wherein the second frequency is about 44 khz and thesecond bandwidth is about 4.2 khz, the ultrasound having a combinedbandwidth of at least about 8 khz.
 4. A system according to claim 1,further comprising clamping means for applying compression to at leastone of the transducers.
 5. A system according to claim 1, wherein thefirst and second frequencies are harmonic frequencies.
 6. A systemaccording to claim 5, wherein the harmonic frequencies are between about100 khz and 350 khz.
 7. A system according to claim 1, furthercomprising one or more other ultrasonic transducers, each of said othertransducers having an additional frequency and an additional ultrasoundband width, wherein the additional bandwidths each overlap with at leastone other of said bandwidths, and wherein each of the additionalfrequencies are different from each other and from the first and secondfrequencies, and wherein the generator means comprises means for drivingthe additional transducer within the additional bandwidths so as toproduce ultrasound within the liquid and with a combined bandwidth thatis greater than any other bandwith.
 8. A system according to claim 7,wherein the additional frequency is a harmonic resonant frequencybetween about 100 khz and 350 khz.
 9. A system according to claim 1, 2,5, 7 or 8 wherein the bandwidths overlap so that, in combination, thetransducers produce ultrasonic energy at substantially all frequencieswithin the combined bandwidth.
 10. A system according to claim 1, 2, 5,7 or 8, wherein the bandwiths overlap so that the transducers andgenerator means produce ultrasonic energy, at each frequency, that iswithin a factor of two of ultrasonic energy produced by the transducersand generator means at any other frequency within the combinedbandwidth.
 11. A system according to claim 1, 2, 5, 7 or 8, wherein thebandwidths overlap so that the transducers and generator means produceultrasonic energy, at each frequency, that is substantially equal to theultrasonic energy produced by the transducers and generator means at anyother frequency within the combined bandwidth.
 12. A system according toclaim 1, wherein the generator means comprises two or more ultrasoundgenerators that are synchronized in magnitude and phase so that there issubstantially zero frequency difference between signals generated by thegenerators.
 13. A system according to claim 12, further comprising FMmeans for generating a master frequency modulated signal to eachgenerator to synchronize the signals from the generators.
 14. A systemaccording to claim 5, wherein the generator means is frequency modulatedover a range of frequencies within the bandwidth of each transducer. 15.A system according to claim 5, wherein the generator means is frequencymodulated over a range of frequencies within the bandwidth of eachtransducer, and wherein the generator means is amplitude modulated overa range of frequencies within the bandwidth of each transducer.
 16. Asystem according to claim 1, further comprising a chamber for holdingthe solution so as to clean or process objects therein.
 17. A systemaccording to claim 16, wherein the chamber comprises a material selectedfrom the group of 316L stainless steel, 304 stainless steel,polytetrafluoroethylene, fluorinated ethylene propylene, polyvinylidinefluoride, perfluoroalkoxy, polypropylene, tantalum, teflon coatedstainless steel, titanium, hastalloy, polyetheretherketone, and mixturesthereof.
 18. A system according to claim 1, wherein each transducercomprises one of the first, second, third or fourth harmonicsfrequencies.
 19. A system according to claim 1, further comprising aliquid, the liquid being responsive to the ultrasound to producecavitation implosion therein.
 20. A system according to claim 19,wherein the liquid comprises one or more chemicals selected from thegroup of solvents, aqueous solutions, and semi-aqueous solutions.